1. Field of the Invention
The present invention relates to a method of manufacturing a thin film EL device and an apparatus for manufacturing the same, and more particularly, it relates a method of forming a light emitting layer of a thin film EL device and an apparatus for forming the same.
2. Description of the Prior Art
In general, a thin film EL device of three-layer configuration as shown in FIG. 6 is known. This thin film EL device is patterned with a transparent substrate 28 of glass and a plurality of strip-like transparent electrodes 23 of ITO (indium tin oxide) disposed in parallel at intervals on the substrate 28, and a first dielectric film 24 made of oxide, such as Al.sub.2 O.sub.3, SiO.sub.2 or TiO.sub.2, or nitride Si.sub.3 N.sub.4 is formed on it. Further, upon it, a light emitting layer (approximately 0.8 .mu.m in thickness) 25 having a composition of base material of ZnS, ZnSe or the like and a trace quantity of Mn added thereto to serve as the center of light emission and a second dielectric film 26 made of the above-mentioned oxide or nitride are formed in this order. This, is patterned with strip-like back electrodes 27 of Al disposed on it in parallel at intervals in a direction perpendicular to the transparent electrodes 23. In the thin film EL device thus configured, selectively applying voltage to the transparent electrodes 23 and the back electrodes 27 causes the light emitting layer 25 at crossing points of both the electrodes 23 and 27 to emit light so that dots in an arbitrary combination emit light. In this way, a desired dot matrix display can be achieved.
In forming the light emitting layer 25 of the thin film EL device, first an additive agent (active element), such as Mn and the like, serving as centers of light emission is mixed with the base material, such as ZnS and the like, at a specified rate, and the mixture is molded and annealed to make a donor pellet. Then, a substance derived from the donor pellet is deposited on the first dielectric film 24 on the surface of the transparent substrate 28 by electron-beam vapor deposition.
In recent years, large-scale display screens have been increasingly developed, and a method in which a plurality of ("two" in this example) vapor substances 80, 90 are deposited on a large-scale substrate 1 is employed as shown in FIG. 5 in order to manufacture a large-area thin film EL device. Specifically, a vapor deposition apparatus 101 is provided with hearths 12, 15, where donor pellets 11, 14 are placed. Surfaces 11a and 14a of the donor pellet 11, 14 are irradiated by electron beams by electron guns 13, 16, and the substances 80, 90 are obtained by evaporation of the donor pellet 11, 14, which are deposited on the substrate 1. During the vapor deposition, the substances 80, 90 are continuously deposited on crystal oscillators 17, 21 placed close to the hearths 12, 15 for monitoring. The vapor deposition apparatus is arranged that the crystal oscillators (referred to as "crystal pieces" herein after) 17, 21 continuously supply signals indicating respective deposition rates, and in accordance with the signals, a controller 30 controls energy of the electron beams so that the substances 80, 90 make equivalent contributions to the deposition on the substrate 1. In this way, making a balanced speed R at which the vapor substances 80, 90 are generated, a light emitting layer formed on the substrate 1 can be uniform in thickness throughout.
In general, the electron beams are scanned while they irradiate the overall areas of the surfaces 11a and 14a so that the whole surfaces of the donor pellets 11, 14 can be uniformly heated and evaporated (see Japanese Unexamined Patent Publication No. 149864/1987).
In manufacturing such a large-area thin film EL device, it is necessary to precisely check uniformity of a thickness of a light emitting layer formed on the substrate 1. Thus, to limit a period of time for the deposition under equivalent energies of the electron beams is unsatisfactory, but what is essential is to monitor the deposition rate of the deposited substances each time a light emitting layer is formed, using the crystal pieces 17, 21 as stated above. However, if the deposition rate is monitored continuously during the deposition, there arise the problem that the lifetime of each of the crystal pieces 17, 21 is expired when a deposition process is almost completed once or even before, because the resultant light emitting layer is thick, approximately 0.8 .mu.m. It is a very difficult task to replace or regulate the crystal pieces for each operation, and this is the obstacle to be overcome when manufacturing thin film EL devices.